It is in the interest of car manufacturers that aftertreatment devices are robust and keep good performance in the terms of delivering legislatively acceptable low emissions for the life time of the vehicle, or at least for 120 000 km driven distance. To fulfill these criteria, an on-board diagnostic system equipped with the method of prediction of catalyst condition is a useful tool to keep optimal operation conditions for the catalyst, and all other parts of after-treatment system.
A method of monitoring a catalyst using a lifetime temperature profile is disclosed in U.S. Pat. No. 5,896,743. U.S. Pat. No. 5,896,743 relates to a monitor for measuring the efficiency of a catalyst located in an exhaust path of an internal combustion engine. The monitor includes a resistive type oxygen sensor positioned in the exhaust path between the engine and the catalyst to detect oxygen concentration of engine emissions, and a temperature sensor positioned proximate to the catalyst for monitoring catalyst temperature. A lifetime temperature profile and a catalyst light off time are generated from a catalyst temperature received from the temperature sensor. There are three comparators, a first comparator for comparing a magnitude of the detected oxygen concentration to a predetermined threshold level; a second comparator for receiving the catalyst light off time and comparing the light off time to a predetermined light off time; a third comparator for receiving the lifetime temperature profile and comparing the lifetime temperature profile to a predetermined temperature profile threshold limit. According to this cited method, time events at which the catalyst temperature exceeds the predetermined limit (800° C.) are counted with different weighing factors depending on a temperature level, thus generating a lifetime temperature profile of the catalyst allowing for estimating the efficiency of a catalyst.
In some investigations, sintering was found to be the one of the primary mechanisms of deactivation of catalysts operating at elevated temperatures. The uniform approach quantifying catalyst thermal ageing in terms of sintering can be applied to both metal oxide catalysts silica, alumina etc. (H. Schaper, et. al. Appl. Catal., 7 (1983) 211;) and supported metal catalysts such as supported noble metal catalysts (E. Ruckenstein et al. J. catal., 29, 224-45 (1973).
EP 1 084 331 B1 relates to a method for monitoring the ability of a catalyst arranged in the exhaust duct of an internal combustion engine, in which a first variable, which is characteristic of the temperature of the catalyst, and a second variable, which is characteristic of the degree of conversion of the catalyst and is dependent on the first variable, are determined by continuous measurements during a heating-up phase of the catalyst, and a change in the dependence of the second variable on the first variable is caused by ageing of the catalyst and is used to monitor the ability function of the catalyst.
EP 0 756 071 B1 relates to a device for determining the abnormal degree of deterioration of catalyst of a catalytic converter arranged in an internal combustion engine exhaust system, comprising measuring a temperature of said catalyst, assuming a temperature of said catalyst on the basis of the engine operating condition, calculating a ratio of a varying value of the measured temperature by said measuring means to a varying value of the assumed temperature by said assuming means; and determining means for determining that the degree of deterioration of said catalyst is abnormal when said ratio is smaller than a predetermined value.
EP 0786586 A2 relates to a device for evaluating performance deterioration of an exhaust gas purifying catalyst, comprising temperature detecting means arranged in an exhaust passage for detecting a temperature of an exhaust gas purifying catalyst, oxygen concentration detecting means arranged in the exhaust passage for detecting a concentration of oxygen contained in an exhaust gas, calculating means for calculating a degree of catalyst performance deterioration in a predetermined period on the basis of the temperature of the catalyst detected by said temperature detecting means and the concentration of oxygen detected by said oxygen concentration detecting means; accumulating means for cumulatively adding the degree of catalyst performance deterioration in said predetermined period to obtain an accumulated value; and evaluating means for evaluating the catalyst performance deterioration on the basis of the accumulated value.
EP 1 544 431 B1 discloses a method for monitoring a status of a catalyst, which allows more precise evaluation of a degree of the thermal ageing of the catalyst (or a degree of the catalyst performance that is still available for exhaust gas after-treatment). According to the mentioned invention, there is provided a method of estimating the efficiency of a catalyst located in the exhaust path and/or downstream of an internal combustion engine comprising the steps of: monitoring a temperature of the catalyst with at least one temperature sensor over the catalyst operating time; calculating an accumulative exposure of the catalyst to thermal ageing conditions with the measured catalyst temperature; applying a predetermined correlation between the accumulative exposure of the catalyst to the thermal ageing conditions and characteristic of the catalyst performance to estimate the catalyst properties; comparing the accumulative exposure of the catalyst to thermal ageing conditions to a predetermined threshold limit of the thermal ageing conditions; and performing indicative measures in case of the actual value of the accumulative exposure of the catalyst to thermal ageing conditions reached and/or exceeded the pre-determined fraction of the pre-determined threshold value, whereby the accumulated exposure of the catalyst to thermal ageing conditions (denoted as TAI, “thermal ageing index”) is calculated using the equation:TAI=Σi[e(−E/RTi)Δti]whereby E/R=activation energy of the sintering process; Ti=catalyst temperature over the i-th time interval over the catalyst lifetime and Δti=length of the i-th interval over catalyst lifetime.
EP 1 544 431 B1 provided a method for estimating the efficiency of a catalyst located in an exhaust path and/or manifold of an internal combustion engine. The method of this invention is based on the assumption that the loss of the efficiency of the high-temperature automotive catalyst is caused, primarily, by thermal ageing. With the parameters of the catalyst temperature recorded with uniform and small time increment over the life of the catalyst, the catalyst efficiency loss can be predicted with high accuracy. With the use of an accurate exhaust gas temperature sensor and/or implied virtual temperature sensor, and knowing a time on-line and/or time on-line during which the catalyst is operated at temperatures above a predetermined threshold temperature, a value of the accumulative exposure of a catalyst to thermal ageing conditions i.e. a thermal ageing index can be calculated using an equation describing the ageing of the catalyst.
Based on the pre-measured correlation between the thermal ageing index and the efficiency of the catalyst, the threshold limiting value of the thermal ageing index is known which corresponds to the threshold (minimally accepted) catalyst efficiency, and also a functional dependence of catalyst efficiency on the thermal ageing index is known. Comparing the actual value and the known threshold value of the thermal ageing index, or applying the predetermined functional correlation between thermal ageing index and catalyst properties as catalyst efficiency, actual catalyst efficiency, or catalyst efficiency loss, or percentage of the catalyst efficiency (resources) left can be accurately estimated.
The method disclosed in EP 1 544 431 B1 assumes that the loss of the efficiency of the high temperature automotive catalyst (TWC, DOC or cDPF) is caused, primarily, by the loss of the supported metal as Platinum (Pt) etc. specific surface area due to the catalyst ageing. It was deduced that changes of the performance characteristics were more consistent with Platinum-related properties. The correlation coefficients between physical properties and CO performance characteristics were calculated, and the strongest correlation was observed between CO performance characteristics, i.e. either light-off temperature shift or tailpipe emissions.
However, the inventors herein have recognized that the above approaches may not accurately predict the conversion efficiency of thermally and hydrothermally aged catalysts, under certain conditions, e.g. when selective catalytic reduction (SCR) catalysts are employed. Further, the above approaches may not accurately predict the efficiency of a catalyst in both ways, the positive way (improvement of activity, or so called activation of the catalyst), and the negative way (decrease of activity, so called ageing). Further, the above-described approaches may not accurately predict actual catalyst efficiency during engine operation and may not be useful for prediction of the actual catalyst efficiency of different types of catalysts with different types of functionality, e.g., as oxidation of CO, and or hydrocarbons, or soot, or catalysts for removal of NOx, such as SCR catalysts.
In one example approach to at least partially address these issues, a method for estimating an efficiency of a catalyst in an exhaust of an engine comprises: ageing the catalyst consecutively at different temperatures for definite time periods; measuring the catalyst conversion efficiency after each ageing step; calculating the ageing factor for each step of the ageing procedure and the accumulative value of the ageing factors for all steps of the ageing procedure based on the formulation AF=Σi(e(−E/RTi)dti) whereby E/R=activation energy of the sintering process, Ti=catalyst temperature over the i-th time interval over the catalyst lifetime and dti=length of the i-th time interval over the catalyst lifetime; estimating the activation energy of the ageing process (Ea/R) for the catalyst; estimating catalyst efficiency correlation factors related to the measured catalyst conversion efficiency for each temperature point and for all ageing conditions; determining a correlation between the catalyst efficiency correlation factors and the accumulative value of the ageing factors; and calculating the conversion efficiency of the aged catalyst based on a predetermined correlation between the accumulative ageing factor and the catalyst efficiency correlation factor.
In this way, an advantageous method for prediction of the performance of aged catalysts, and in particular for predicting a NOx conversion efficiency value for aged SCR catalysts is provided. Furthermore, in this approach, a safe index for catalyst operation and a maximum safe temperature operation may be obtained so that aged catalysts are able to deliver optimal performance. Further, such an approach may increase accuracy in the prediction of the actual catalyst efficiency during engine operation for different types of catalysts with different types of functionality, e.g., as oxidation of CO, and or hydrocarbons, or soot, or catalysts for removal of NOx, e.g., in a SCR catalyst.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.